Method of making impregnated coils for electromagnetic apparatus and the product thereof



Dec. 8, 1964 A. J. LIBERMAN 3,160,840

METHQD OF MAKING IMPREGNATED COILS FOR ELECTROMAGNETIC APPARATUS AND THE PRODUCT THEREOF Filed Sept. 14. 1960 INVENTORL fimwl Zz'emm 0 BY W W'TW CH ifm/zazgs- United States Patent 3,160,840 METHOD OF MAKENG IMPREGNATED CGILS FQR ELECTRGMAGNETEC APPARATUS AND THE PRODUCT THEREQF Arnold J. Liherman, Chicago, BL, assignor to Advance Transformer Co., (Ihicago, 111., a corporation of Illinois Filed Sept. 14, 1966, Ser. No. 55,957 3 Claims. (ill. 336-96) This invention relates generally to electromagnetic apparatus and more particularly, relates to improvements in electrical coils for such apparatus which are impregnated with a suitable insulating compound and to an improved method of manufacturing such impregnated coils for electromagnetic apparatus.

The advantages to be derived from the invention are not necessarily limited to any special type of electro magnetic apparatus employing electrical coils which are impregnated with a suitable insulating compound; however, the invention is especially advantageous in connection with electrical ballasting apparatus commonly used in the operation of gaseous discharge devices, such as fluorescent lamps. For this reason and to contribute to a full understanding and appreciation of the advance in the arts and sciences achieved by the invention, this disclosure will be prefaced by a discussion of the peculiar problems heretofore prevalent in the manufacture of electrical ballasting apparatus for operating fluorescent lamps which the invention solves satisfactorily.

As Well known, a gaseous discharge device such as a fluorescent lamp exhibits negative resistance characteristics after ignition thereof when electrical current is conducted through the ionized gases in the lamp. In order to provide the ignition and operating voltages of such devices for the dual purpose of limiting the amount of current drawn by the device subsequent to its ignition and to provide desired voltages for proper ignition and/or operation of the device, a transformer is used in the fluorescent lamp circuit which generally is installed in the fixture which holds the fluorescent lamp. Such a transformer is known in the trade as a ballast and is usually characterized as having high leakage reactance to accomplish the dual purposes aforesaid. Ballasts are basically chokes and/ or transformers usually including magnetic core structures mounting a plurality of electrical coils connected with one or more condensers and/ or resisters, chokes and the like in desired combination. The electrical components of the ballasts are disposed in canisters to enable them readily to be handled and installed in the fluorescent lamp fixture. The canisters are filled with a suitable potting compound which serves to prevent shifting of the electrical components of the ballast, to support and protect the electrical components, electrical leads and terminals, and to assist in dissipation of heat generated in the ballast for various reasons during operation. Further, such potting compounds help to reduce noise occasioned by vibrations in the ballast during use thereof.

Transformer ballasts usually also require satisfactory impregnation of both the electrical coils and the magnetic core with a suitable insulating compound in order to reduce noise and to prevent voltage breakdown and moisture absorption. It is in connection with satisfactory impregnation of the electrical coil especially that here tofore prevalent and long suffered problems for the first time have been successfully solved by means of the herein invention.

Electrical coils of these ballasts are usually intended to be subjected to relatively high voltages and are formed of a desired number of turns of suitable gauge enamelcoated wire wound on paperboard or fiberboard forms.

The electrical wire is wound in layers alternating with sheet insulating material. It has been common to use fibrous material such as paper and more expensive acetate cellulose, in each instance, the sheet insulating material being of suitable gauge so as to have enough body strength to support the rows or layers of Wire and suitable dielectric strength to withstand the voltages involved. Obviously, heavier gauges of sheet insulating material increase the cost and size of the product. Further, it will be appreciated that the tightly wound electrical coils have their wire turns intimately and closely engaged with the sheet insulating material so that access to the interior spaces or reaches of the coil by fluids is highly restricted. This also applies with respect to the interstices and crevices of the magnetic core of a ballast, but with considerably less restriction. For instance, in some instances, it has been possible to realize penetration of the core with compounds merely by immersing the core in a bath of the compound, however, this has not been possible in the case of electrical coils.

A common technique heretofore employed has been to assembly the coils and core structure of the ballast to a desired stage of completion for the ballast and thereafter, impregnate the assemblage with a varnish or impregnating compound such as a combination asphaltic material and waxes which has the desired high temperature melt or softening point and reduction in viscosity of the compound beyond the said point. The desired low viscosity of the impregnating compound is realized as a result of running of the waxes selectively used, said waxes usually being of the micro-crystalline type. One procedure of impregnating is to preheat the transformer core and coil assemblage to a temperature approximating the melt or softening point of the impregnating compound or even be yond that temperature point, and thereafter, dipping or immersing the assemblage in a bath of molten compound. However, this practice does not provide for proper and satisfactory impregnation of the coils because the molten compound cannot penetrate through the reaches of the coils. Another technique has been to preheat the core and coil assembly and by means of suitable vacuumimpregnating equipment, evacuate a chamber in which the assembly has been placed and introduce molten impregnating compound into the evacuated chamber to be drawn into the interstices and reaches of the core and the coils concurrently. This technique normally required sev cral cycles of the evacuating and introduction of impregnating compound in order to assure proper impregnation of the coils. Still another technique has utilized vacuum impregnation of the core and coils assemblage with a varnish which must thereafter be cured in a baking chamber. In all instances, the concern for obtaining a properly impregnated core and coil assemblage, and especially impregnation of the coils, required special equipment and personnel. Space, expense and time were increased over ordinary dipping.

Subsequently, the impregnated core and coil assembly is encapsulated in a suitable container filled with some type of potting compound which usually is a mixture of an asphaltic material such as pitch and an inert material such as silica, diatomaceous earth, and the like. These compounds also may be polyester resins combined with silica or epox -coal tar materials to which silica has been added. Using a potting compound of sufficiently low viscosity at a temperature above its melt or softening point, it is possible for the liquid potting compound to penetrate the interstices and crevices of the transformer core structure, however, such compounds will not penetrate properly into the reaches of the coils merely by such dipping practices and without the use of vacuum impregnating equipment.

As a result of demands for new fluorescent lamp installations and the normal need for repair and servicing of existing lamp installations, the transformer ballast industry is required to produce monumental numbers of such electrical ballasting devices. Competition in this id dustry is very keen so that mass-production and low cost techniques of manufacture must be employed in order to attain the required volume of production and to compete favorably and economically. The cost of labor is an important factor in achieving favorable competitive advantage in this industry. As a consequence of the impregnating techniques employed as previously enumerated and possible other techniques not mentioned, it has been necessary to interrupt an otherwise continuous assembly operation of such ballasts properly and satisfactorily to obtain impregnation of the coils as well as the magnetic core mounting the coils. It is only after such impregnation procedures have been. completed that the core and coil assembly can be encapsulated as explained and therefore, to this extent the cost of manufacture of the ballasting devices has been increased as a result of the additional handling time and money required to impregnate the core and coil.

It should be understood that although the mentioned techniques of vacuum impregnating the magnetic core and electrical coils simultaneously are more economical and expedient than doing this in two steps, the practice is necessitated by the fact that the electrical coils can only be vacuum impregnated and consequently, the normal continuous assembly of such ballasts was still required to be interrupted with resulting increased handling costs and expenditure of time.

Accordingly, it is a principle object of the invention to provide a novel method of manufacturing an impregnated electrical coil of the character described which will substantially eliminate the disadvantages herein enumerated.

An ancillar ob ect is to rovide an electrical coil construction of the character described which operates quietly, and achieves the desired insulation against voltage breakdown and moisture absorption.

An important object of the invention is to provide an improved method of manufacturing an impregnated coil of the characted described which eliminates such prior vacuum impregnating procedures for manufacturing such coils with attendant substantial reduction in the amount of time and expense heretofore expended as a result of interrupting of normal continuous production for vacuum impregnation of the coils.

An important object of the invention is to provide a method of manufacturing such electrical coils which enables a satisfactorily impregnated coil to be produced concurrently with completion of winding of the coils and using heretofore conventional coil winding equipment, if desired.

Another important object of the invention is to provide an electrical coil structure of the character described which may or may not include a magnetic core structure assembled therewith in which the coil has the wire turns thereof wound in layers with sheet insulating material interleaved between the wire layers, said sheet insulating material having been pro-treated prior to winding of the coil with an impregnating compound without substantially affecting the desirable flexibility thereof whereby upon completion of the winding of the coil using said pre-treated sheet insulating material, an impregnated electrical coil is produced which is the equivalent of the heretofore conventional vacuum-impregnated electrical coil insofar as desirable operating characteristics are concerned.

Another object of the invention is to provide a method of manufacturing an impregnated magnetic core and coil assembly for transformer ballasts which is characterized by the elimination of heretofore conventional vacuumimpregnating procedures for obtaining satisfactory impregnation of either the magnetic core or the coil or both.

In addition to the objects of the invention heretofore specified, other objects and advantages derived therefrom will become apparent or may occur to the skilled artisan having the benefit of this disclosure. The invention further is characterized by the simplicity and economy of procedures which may be employed to derive the benefits thereof, all of which were heretofore unobvious to the skilled artisan because of the peculiar problems attendant the successful, economical and commercially favorable production of the structures with which the invention is concerned. It is contemplated that minor variations in the steps of the method concerned and in the structures hereinafter set forth in detail and illustrated in the accompanying drawing may be made without departing from the scope or sacrificing any of the advantages of the invention.

In the drawing:

FIG. 1 is a perspective view of a representative ballast construction with a portion of the canister thereof broken away partially to reveal a magnetic core and coil assemblage encapsulated therein, the ballast illustrate being of a conventional construction with the exception of the means by which impregnation of the core and coil assemblage has been accomplished in accordance with the invention.

FIG. 2 is a perspective view of an impregnated electrical coil embodying the invention.

FIG. 3 is a diagrammatical view showing the manner of winding the coil in accordance with the invention.

FIG. 4 is a fragmentary sectional view taken through the coil along the line 44 of P16. 3 and in the general direction indicated.

FIG. 5 is an enlarged fragmentary sectional view showing a detailed feature of the coil embodying the inr vention.

FIG. 6 is a fragmentary sectional view taken trans versely through one form of the sheet insulating material and FIG. 7 is a fragmentary sectional view taken transversely through another form of the sheet insulating material used in the electrical coil embodying the invention.

Referring now to the drawing, the reference character 2i) represents a typical transformer ballast used to ignite and operate fluorescent lamps. Said ballast 20 includes a transformer core and coil assemblage designated generally 22 which is placed in a metallic canister designated generally 24, the entire canister being filled with a potting compound, as at 26. In the typical construction 20, electrical leads (not shown) protrude through openings in end walls of the canister. Also not shown, but readily understood to be commonly used in this type of construction are condensers, resistors and/or other electrical components which may be encased with the assemblage 22 in the canister.

The metal canister 24- includes a shallow lower receptacle or box 28 open at its top and a cover member 30 suitably designed to cooperate with the box 28 to provide mounting flange formations designated generally 32 along upper edges of the canister and adjacent opposite end walls 34 of the box 28. The precise construction of the canister is immaterial to the invention. The assemblage 22 on the interior of the ballast includes an iron core 35 and various electrical coils 38. FIG. 1 does not illustrate any precise manner of connecting the electrical components of a typical ballast 20 or all of the electrical components thereof because this is not material to the invention.

For purposes of this disclosure, the metal core 36 is comprised generally of a plurality of individual electrical steel laminations of suitable configuration and size stacked and clamped together in a rigid assembly. The coils 38 are supported on a central winding leg 40 of the core in suitable windows 2 provided during manufacture of the core. it will be appreciated that the precise size, configuration and structural details of the core 36 may vary considerably without affecting the invention. The close face-to-face contact of adjacent laminations restricts access to the interior reaches of the core so that satisfactory impregnation of the core requires the exercise of care and attention.

In practice, the coil 38 shown in FIG. 2 is one of several which would be severed from a longer coil segment or stick ordinarily wound by means of well-known coil winding equipment on a paperboard or fibreboard form 44 of square cross-section and provided with the leads 46 for connection thereof to other electrical components, as desired. Generally, the coil 38 will include a desired number of turns of wire 48 wound in layers and alternating with sheet insulating material 50, as seen in FIG. 4. According to the invention the sheet insulating material comprises individual sheets which have been treated with a suitable impregnating compound prior to winding of the coil 38. Thus, upon winding of the coil 38 by conventional means, but using the pie-treated sheets 50, a fully and satisfactorily impregnated coil is obtained.

Referring to FIG. 6, one form of the sheet 50 is illustrated in which a web 52 of paper or acetate cellulose has been treated to provide a substantially uniform coating or layer 54 of the impregnating compound on each surface thereof. This may be done by dipping or running the web 52 through a bath of the compound or by other techniques such as spraying, calendaring or the like. It is believed that small quantities of the compound are absorbed by the web 52 and a thin layer or film 54 is formed on the surfaces of the web. Taking into account the thickness of each of the films or layers 54, the gauge of the web 54 employed will vary proportionally with the size of the wire so that large gauges of wire will re quire greater thicknesses for the web 52. Consequently, the web 52 of paper or acetate cellulose used is lesser in thickness than heretofore would have been used with sheet insulating paper which had not been treated in accordance with the invention. This is possible notwithstanding the heretofore recognized requirement of suitable gauge of sheet insulating material having sufficient body strength to support the turns of wire 48. The layers or films 54 not only provide the necessary insulation but also contribute to sufiicient stiffness and body strength of the sheet material 50 to support the layers of wire of the coil. Consequently, the invention enables the use of the more inexpensive paper sheet insulating material of lesser gauges than heretofore would be feasible, even for electrical coils intended for use in higher voltage environments.

Referring to FIG. 7, another form of sheet insulating material 50' is illustrated in which a web of paper has been impregnated with the impregnating compound 54 throughout its extent. In this form of the invention, the sheet material 50 has the impregnating compound absorbed therein and a residual film of compound is formed on the surfaces of the web. The gauge of the sheet material used in substantially the same as the gauge of the impregnated sheet material 50'. This form of the invention permits lesser gauges of sheet insulating material to be used because the impregnating compound also stitfens the resulting product.

The coil 38 embodying the invention has been successfully achieved using an asphaltic wax composition which has certain desirable physical properties. The usual asphaltic compounds were employed to which were added high-temperature waxes so that the melt or softening point of the composition ranged from about 250260 F. Characteristic of the composition was the high drop in viscosity at higher temperatures, whereas at normal room temperatures, for instance, the material desirably is solid. The sheet materials 50 or 50 can be conveniently stored, impregnating composition remaining solid, but slightly tacky and soft so that there is no problem of separating sheets which have been in face-to-face contact for long periods of time. If necessary interleaving sheets may be used and discarded as the coils are wound. An important characteristic of the impregnated coil 38 can be seen from FIG. 5 wherein the turns of wire 48 are shown somewhat embedded in the layers 54 of the impregnating composition between which they are sandwiched by the reference character 56. Notably then is the factor that the somewhat soft composition permits the wires 48 to be substantially encased in the impregnating compound, so to speak, upon completion of the winding thereof and subsequent shaping of the coil in accordance with conven tional practices. Lateral movement of wire turns is resisted by this structure. The sheet material may vary in thickness from approximately .001 to .003 inch where used with wires ranging from #34 to #20 gauge (check scale). Noteworthy is the factor that the sheets 50 or 5% retain their flexibility and can be employed in conjunction with conventional coil winding machines.

To further appreciate the benefits derived from the invention, the method of making such core and coil assemblages 22 as a result of using the invention will be dis cussed. The coils 38 are manufactured using the pretreated sheet insulating material 50 or 50' in elongate coil forms from which the desired coils are separated and shaped using conventional techniques. The coils are assembled on the central winding leg 40 and the central winding leg assembled in the shell structure of the core. At this stage it had heretofore been required to vacuum impregnate the core and coil assemblage, but the invention eliminates this step entirely. The core and coil assemblage is now ready to be potted in the canister directly, or could be pre-heated and dipped into a bath of potting compound. Since the coils 38 already have been impregnated, it is possible to use available potting compounds having sufficiently low viscosity in their molten state so that the compound will penetrate the interstices and reaches of the core. It also is feasible to impregnate the core at this stage with the same impregnating compound used on the sheet material since this compound is selected to exhibit the characteristic of low surface tension and low viscosity suitable for penetrating throughout the steel of the core. In any event, the heretofore timely and costly operation of vacuum impregnation of the electrical coils is entirely eliminated so that the manufacture of the ballast 20 can continue uninterrupted with attendant substantial savings in the cost engendered by vacuum impregnation heretofore required.

It is believed the invention has been described in sufficient detail to enable the skilled artisan to understand and practice the same. In the claims, the terms impregnated coil, unless otherwise qualified, will be used to designate the structure wherein the sheet insulating material thereof either is coated as shown in FIG. 6 or absorbed, as shown in FIG. 7. The terms impregnating composition will be used to designate both the compound which is absorbed by or coated on the sheet insulating material. Also, the terms impregnated sheet insulating material will be used to include both of the structures 50 and 50' unless otherwise qualified.

What it is desired to secure by Letters Patent of the United States is:

1. In the manufacture of a ballast for starting and operating gaseous discharge devices including a magnetic iron core formed of individual laminations of electrical steel assembled together and having at least one electrical coil mounted upon said core, the core and coil being disposed within a canister and immersed in a conventional asphaltic based potting compound, the said core and coil being impregnated with an electrical insulating compound compatible with said potting compound; a method of impregnating said core and coil comprising the steps of, initially covering at least one surface of a sheet of insulating material with an impregnating compound having an asphaltic base including a wax compatible with said potting compound, to be normally solid at room temperature, to have a sharp drop in viscosity at its softening temperature san a s-o and to have a softening point between 250 and 260 degrees Fahrenheit, winding layers of coated electrical wire upon a form at room temperature with said treated sheet interleaved between adjacent layers of said wire to form said coil, mounting said coil upon said core and immersing the said core and coil in hot molten potting compound to cause the impregnating compound of the sheet to flow to fill the unoccupied space of the coil, a portion of said potting compound simultaneously entering the interstices of the core between the laminations thereof.

2. The method as claimed in claim 1 in which said impregnating compound is substantially identical to said conventional potting compound but free from any mineral filler.

3. In a ballast for starting and operating gaseous discharge devices including a magnetic iron core formed of individual laminations of electrical steel assembled together and having at least one electrical coil mounted upon said core, the core and coil being disposed within a canister and immersed in a conventional potting compound; the electric coil comprising, a plurality of coated electrical wire layers wound on a winding form and a plurality of layers of sheet insulating material interleaved between adjacent layers of said wire, an asphaltic based References Cited in the file of this patent UNITED STATES PATENTS 486,926 Buck Nov. 29, 1892 1,486,648 Evans Mar. 11, 1924 1,874,722 Turner Aug. 30, 1932 1,998,827 Worrell et al. Apr. 23, 1935 2,195,233 Boyer Mar. 26, 1940 2,246,159 Work et al. June 17, 1941 2,311,450 Marsh Feb. 16, 1943 2,334,584 Rich NOV. 16, 1943 2,692,423 Elliott Oct. 26, 1954 2,863,796 Zack et al. Dec. 9, 1958 2,915,723 Wennerberg Dec. 1, 1959 2,997,528 Kessel et a1 Aug. 22, 1961 

3. IN A BALLAST FOR STARTING AND OPERATING GASEOUS DISCHARGE DEVICES INCLUDING A MAGNETIC IRON CORE FORMED OF INDIVIDUAL LAMINATIONS OF ELECTRIC STEEL ASSEMBLED TOGETHER AND HAVING AT LEAST ONE ELECTRICAL COIL MOUNTED UPON SAID CORE, THE CORE AND COIL BEING DISPOSED WITHIN A CANISTER AND IMMERSED IN A CONVENTIONAL POTTING COMPOUND; THE ELECTRIC COIL COMPRISING, A PLURALITY OF COATED ELECTRICAL WIRE LAYERS WOUND ON A WINDING FORM AND A PLURALITY OF LAYERS OF SHEET INSULATING MATERIAL INTERLEAVED BETWEEN ADJACENT LAYERS OF SAID WIRE, AN ASPHALTIC BASED INSULATING COMPOSITION DISPOSED ON THE SURFACE OF SAID SHEET INSULATING MATERIAL BETWEEN SAID LAYERS OF WIRE, SAID INSULATING COMPOSITION HAVING A SOFTENING TEMPERATURE BETWEN 250 AND 260 DEGREES FAHRENHEIT AND A SHARP DECREASE OF VISCOSITY AT SAID SOFTENING TEMPERATURE, SAID INSULATING COMPOSITION FILLING SUBSTANTIALLY ALL SPACES WITHIN SAID COIL UNOCCUPIED BY SAID WIRE SUBSEQUENT TO IMMERSION IN SAID CONVENTIONAL POTTING COMPOUND AND BEING COMPATIBLE THEREWITH. 